System and method for locating an anomaly ahead of a drill bit

ABSTRACT

A method and a system are provided for allowing determination of a distance from a tool to anomaly ahead of the tool. The apparatus for performing the method includes at least one transmitter and at least one receiver. An embodiment of the method includes transmitting electromagnetic signals from the at least one transmitter through the formation surrounding the wellbore and detecting voltage responses at the at least one receiver induced by the electromagnetic signals. The method includes calculating apparent conductivity or apparent resistivity based on a voltage response. The conductivity or resistivity is monitored over time, and the distance to the anomaly is determined from the apparent conductivity or apparent resistivity values.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part of U.S. applicationSer. No. 10/701,735 filed on Nov. 5, 2003, which is incorporated hereinby reference. Applicants therefore, claim priority based on the filingdate of U.S. application Ser. No. 10/701,735.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

FIELD OF THE INVENTION

The present invention relates to a method and system for locating ananomaly and in particular to finding the location of a resistive orconductive anomaly in a formation surrounding a borehole in drillingapplications.

BACKGROUND OF THE INVENTION

In logging while drilling (LWD) geo-steering applications, it isadvantageous to detect the presence of a formation anomaly ahead of oraround a bit or bottom hole assembly. There are many instances where“Look-Ahead” capability is desired in LWD logging environments.Look-ahead logging is to detect an anomaly at a distance ahead of thedrill bit. Some look-ahead examples include predicting an over-pressuredzone in advance, or detecting a fault in front of the drill bit inhorizontal wells, or profiling a massive salt structure ahead of thedrill bit. While currently available techniques are capable of detectingthe presence of an anomaly, they are not capable of determining thelocation of the anomaly with sufficient depth or speed, they are notcapable of detecting an anomaly at a sufficient distance ahead of a bitor bottom hole assembly.

In formation evaluation, the depth of investigation of most loggingtools, wire line or LWD has been limited to a few feet from theborehole. One such tool is disclosed in U.S. Pat. No. 5,678,643 toRobbins, et al. U.S. Pat. No. 5,678,643 to Robbins, et al. discloses anLWD tool for locating an anomaly. The tool transmits acoustic signalsinto a wellbore and receives returning acoustic signals includingreflections and refractions. Receivers detect the returning acousticsignals and the time between transmission and receipt can be measured.Distances and directions to detected anomalies are determined by amicroprocessor that processes the time delay information from thereceivers. As set forth above, the depth of investigation facilitated bythe tool is limited.

Another technique that provides limited depth of investigation isdisclosed in U.S. Pat. No. 6,181,138 to Hagiwara. This technique forlocating an anomaly utilizes tilted coil induction tools and frequencydomain excitation techniques. In order to achieve a depth ofinvestigation with such a tool, a longer tool size would be required.However, longer tools generally result in poor spatial resolution.

In order to increase depth capabilities, transient electromagnetic (EM)methods have been proposed. One such method for increasing the depth ofinvestigation is proposed in U.S. Pat. No. 5,955,884 to Payton, et al.The tool disclosed in this patent utilizes electric and electromagnetictransmitters to apply electromagnetic energy to a formation at selectedfrequencies and waveforms that maximize radial depth of penetration intothe target formation. In this transient EM method, the current isgenerally terminated at a transmitter antenna and temporal change ofvoltage induced in a receiver antenna is monitored. This technique hasallowed detection of an anomaly at distances as deep as ten to onehundred meters. However, while Payton discloses a transient EM methodenabling detection of an anomaly, it does not provide a technique fordetecting anomalies ahead of a drill bit.

Other references, such as PCT application WO/03/019237 also disclose theuse of directional resistivity measurements in logging applications.This reference uses the measurements for generating an image of an earthformation after measuring the acoustic velocity of the formation andcombining the results. This reference does not disclose a specificmethod for determining distance and direction to an anomaly.

When logging measurements are used for well placement, detection oridentification of anomalies can be critical. Such anomalies may includefor example, a fault, a bypassed reservoir, a salt dome, or an adjacentbed or oil-water contact. It would be beneficial to determine both thedistance and the direction of the anomaly from the drilling site.

Tri-axial induction logging devices, including wire-line and LWD devicesare capable of providing directional resistivity measurements. However,no method has been proposed for utilizing these directional resistivitymeasurements to identify the direction to an anomaly.

Accordingly, a new solution is needed for determining the distance froma tool to an anomaly. Particularly such a solution is needed for lookingahead of a drill bit. Furthermore, a real time solution having anincreased depth of analysis is needed so that the measurements can beimmediately useful to equipment operators.

SUMMARY OF THE INVENTION

In one aspect, an embodiment of the present invention is directed to amethod for determining a distance to an anomaly in a formation ahead ofa wellbore. The method is implemented using a device including at leastone transmitter and at least one receiver. The method includescalculating apparent conductivity based on a voltage response. Theconductivity is monitored over time, and the distance to the anomaly isdetermined when the apparent conductivity reaches an asymptotic value.

In a further aspect, a method for determining a distance to an anomalyin a formation surrounding a wellbore is provided. The method isaccomplished using a device with at least one transmitter fortransmitting electromagnetic signals and at least one receiver fordetecting responses. The method includes transmitting an electromagneticsignal with the transmitter in the direction to the anomaly. At thereceiver, the voltage response is measured, and an apparent conductivityis monitored over time based on the voltage response. The distance to ananomaly is located based on the variation of the apparent conductivityover time.

In yet another aspect, an embodiment of the invention provides a methodfor determining a distance to an anomaly ahead of a wellbore. The methodis implemented using a device including at least one transmitter fortransmitting electromagnetic signals and a receiver for detectingresponses. The method includes transmitting an electromagnetic signalwith the transmitter in the direction of the anomaly. A voltage responseis measured over time, and the response is utilized to calculate anapparent conductivity over a selected time span. A time at which theapparent conductivity deviates from a constant value is determined, andthis time may by utilized to ascertain the distance which an anomaly isahead of a wellbore.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described in detail below with reference to theattached drawing figures, wherein:

FIG. 1 is a block diagram showing a system in accordance with embodimentof the invention;

FIG. 2 is a flow chart illustrating a method in accordance with anembodiment of the invention;

FIG. 3 is a graph illustrating directional angles between toolcoordinates and anomaly coordinates;

FIG. 4A is a graph showing a resistivity anomaly in a tool coordinatesystem;

FIG. 4B is a graph showing a resistivity anomaly in an anomalycoordinate system;

FIG. 5 is a graph illustrating tool rotation within a borehole;

FIG. 6 is a graph showing directional components;

FIG. 7 is a schematic showing apparent conductivity with a coaxial tool;

FIG. 8 is a schematic showing apparent conductivity with a coplanartool.

FIG. 9 is a schematic showing apparent conductivity with a coaxial tool;and

FIG. 10 is a schematic showing apparent conductivity with a coplanartool.

FIG. 11 and FIG. 12 are plots showing the voltage from the coaxialV_(zz)(t), coplanar V_(xx)(t), and the cross-component V_(xx)(t)measurements for L=1m, for O=30°, and at salt distance D=10m and D=100mrespectively;

FIG. 13 is a plot showing the apparent dip (O_(app)(t) for the L=1m toolassembly when the salt face is D=10m away and at the approach angle ofθ=30°,

FIG. 14 is a plot showing the apparent conductivity (σ_(app)(t) fromboth the Coaxial (V_(zz)(t) and the coplanar (V_(xx)(t)) responses;

FIG. 15 is a plot showing the ratio, σ_(app-coplanar)(t)/σ_(coaxial)(t),that is available without cross-component V_(xx)(t) measurements;

FIG. 16 is a plot showing the apparent dip θ_(app)(t) for the L=1m toolassembly when the salt face is D=10m away, but at different anglesbetween the tool axis and the target;

FIGS. 17, 18, and 19 are plots comparing the apparent dip θ_(app)(t) fordifferent salt face distances (D) and different angles between the toolaxis and the target,

FIGS. 20-22 are plots showing the voltage response for different valuest and L in where σ₁-σ₂,

FIGS. 23 and 24 are plots illustrating to voltage response for differentvalues of t and L in FIGS. 23-24, where σ₁=σ₂

FIG. 25 is a plot showing the voltage response of the L=01mtransmitter-receiver offset coaxial tool at different distances;

FIG. 26 is a plot showing the voltage data of FIG. 25 plotted in termsof apparent conductivity;

FIG. 27 is a plot illustrating the apparent conductivity in a two-layermodel where σ₁=1 S/m (R₁=1ohm−m) and σ₂=1 S/m (R₂=1ohm−m)

FIG. 28 compares the σ_(app)(t) plot of FIGS. 26 and 27 for L=1m andd=1m where the resistivity ratio R₁/R₂ is 10:1 in FIG. 26 and 1:10 inFIG. 27

FIG. 29 is a plot comparing the σ_(app)(t) plots for d−1m at differentspacings L;

FIG. 30 is a plot comparing the σ_(app)(t) plots for d=1m and L=1m atdifferent resistivity ratios;

FIG. 31 is exemplary σ_(app)(t) plots for d−1m and L=1m for differentresistivity ratios of the target layer 2 while the local conductivity(σ₁) is fixed at 1 S/m (R₁=1 ohm-m).

FIG. 32 shows the apparent conductivity at large values of t when σ₁ isfixed at 1 S/m;

FIG. 33 is a plot illustrating that the conductivity at large values oft (as t approaches infinity) can be used to estimate the conductivity(σ₂) of the adjacent layer when the local conductivity (σ₁) near thetool is known;

FIG. 34 is a plot showing that the transition time at which the apparentconductivity (σ_(app)(t)) starts deviating from the local conductivity(σ₁) towards the conductivity at large values of t depends on d and L,;

FIG. 35 is a plot illustrating that the distance (d) can be estimatedfrom the transition time (t_(c));

FIG. 36 is a plot illustrating that the apparent conductivity(σ_(app)(z; t)) for different distances (d) in both the z and tcoordinates may serve as an image presentation of the transient data aL=01m tool;

FIG. 37 is a plot of the voltage response of the L=01 m(transmitter-receiver offset) coaxial tool at different distances (d) asa function of t;

FIG. 38 is a plot of the same voltage data of FIG. 37, plotted in termsof the apparent conductivity (σ_(app)(t));

FIG. 39 is a σ_(app)(t) plot of the coaxial transient response in thetwo-layer model of FIG. 9 for an L=01 m tool at different distances (d),except that the conductivity of the local layer (σ₁) is 1 S/m (R₁−1ohm-m) and the conductivity of the target layer (σ₂) is 0.1 S/m (R₂=10ohm-m);

FIG. 40 is a plot comparing the σ_(app)(t) plots of FIG. 38 and FIG. 39for L=01 m and d=01 m;

FIG. 41 is a plot illustrating that the late time conductivity may haveto be approximated by σ_(app)(t=1 second), which slightly depends on d;

FIG. 42 compares the σ_(app)(t) plots for d=1m but with differentspacing L.

FIG. 43 is a plot illustrating that the late time conductivity definedat t−1 second, depends on the distance (d);

FIG. 44 is a plot comparing the σ_(app)(t) plots for d=05 m and L=01 mfor different resistivity ratios;

FIG. 45 is a plot showing examples of σ_(app)(t) plots for d=05 m andL=01 m for different resistivity ratios while the target resistivity isfixed at R₂=1 ohm-m;

FIG. 46 is a plot illustrating that the late time apparent conductivityat t=1 second is determined by the local layer conductivity;

FIG. 47 is a plot illustrating that the late time conductivity(σ_(app)(t→∞)) at T=1 second can be used to estimate the conductivity ofthe adjacent layer (σ₂) when the local conductivity near the tool (σ₁)is known;

FIG. 48 is a plot illustrating that the transition time (t_(c)) at whichthe apparent conductivity starts deviating from the local conductivity(σ₁) toward the late time conductivity depends on d, the distance of thetool to the bed boundary, as shown in for a L=01 m tool;

FIG. 49 is a plot illustrating that the distance (d) can be estimatedfrom the transition time (t_(c)), when L=01 m;

FIG. 50 is a plot of the apparent conductivity (σ_(app)(z; t) in both z−and t− coordinates.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention relate to a system and method fordetermining distance and direction to an anomaly in a formation within awellbore. Both frequency domain excitation and time domain excitationhave been used to excite electromagnetic fields for use in anomalydetection. In frequency domain excitation, a device transmits acontinuous wave of a fixed or mixed frequency and measures responses atthe same band of frequencies. In time domain excitation, a devicetransmits a square wave signal, triangular wave signal, pulsed signal orpseudo-random binary sequence as a source and measures the broadbandearth response. Sudden changes in transmitter current cause signals toappear at a receiver caused by induction currents in the formation. Thesignals that appear at the receiver are called transient responsesbecause the receiver signals start at a first value and then decay orincrease with time to a constant level. The technique disclosed hereinimplements the time domain excitation technique.

As set forth below, embodiments of the invention propose a generalmethod to determine a direction to a resistive or conductive anomalyusing transient EM responses. As will be explained in detail, thedirection to the anomaly is specified by a dip angle and an azimuthangle. Embodiments of the invention propose to define an apparent dip(θ_(app)(t)) and an apparent azimuth (φ_(app)(t)) by combinations oftri-axial transient measurements. An apparent direction ({θ_(app)(t),φ_(app)(t)}) approaches a true direction ({θ, φ}) as a time (t)increases. The θ_(app)(t) and φ_(app)(t) both initially read zero whenan apparent conductivity σ_(coaxial)(t) and σ_(coplanar)(t) from coaxialand coplanar measurements both read the conductivity around the tool.The apparent conductivity will be further explained below and can alsobe used to determine the location of an anomaly in a wellbore.

FIG. 1 illustrates a system that may be used to implement theembodiments of the method of the invention. A surface computing unit 10may be connected with an electromagnetic measurement tool 2 disposed ina wellbore 4 and supported by a cable 12. The cable 12 may beconstructed of any known type of cable for transmitting electricalsignals between the tool 2 and the surface computing unit 10. One ormore transmitters 16 and one are more receivers 18 may be provided fortransmitting and receiving signals. A data acquisition unit 14 may beprovided to transmit data to and from the transmitters 16 and receivers18 to the surface computing unit 10.

Each transmitter 16 and each receiver 18 may be tri-axial and therebycontain components for sending and receiving signals along each of threeaxes. Accordingly, each transmitter module may contain at least onesingle or multi-axis antenna and may be a 3-orthogonal componenttransmitter. Each receiver may include at least one single or multi-axiselectromagnetic receiving component and may be a 3-orthogonal componentreceiver.

The data acquisition unit 14 may include a controller for controllingthe operation of the tool 2. The data acquisition unit 14 preferablycollects data from each transmitter 16 and receiver 18 and provides thedata to the surface computing unit 10.

The surface computing unit 10 may include computer components includinga processing unit 30, an operator interface 32, and a tool interface 34.The surface computing unit 10 may also include a memory 40 includingrelevant coordinate system transformation data and assumptions 42, adirection calculation module 44, an apparent direction calculationmodule 46, and a distance calculation module 48. The surface computingunit 10 may further include a bus 50 that couples various systemcomponents including the system memory 40 to the processing unit 30. Thecomputing system environment 10 is only one example of a suitablecomputing environment and is not intended to suggest any limitation asto the scope of use or functionality of the invention. Furthermore,although the computing system 10 is described as a computing unitlocated on a surface, it may optionally be located below the surface,incorporated in the tool, positioned at a remote location, or positionedat any other convenient location.

The memory 40 preferably stores the modules 44, 46, and 48, which may bedescribed as program modules containing computer-executableinstructions, executed by the surface computing unit 10. The programmodule 44 contains the computer executable instruction necessary tocalculate a direction to an anomaly within a wellbore. The programmodule 46 includes the computer executable instructions necessary tocalculate an apparent direction as will be further explained below. Theprogram module 48 contains the computer executable instructionsnecessary to calculate a distance to an anomaly. The stored data 46includes data pertaining to the tool coordinate system and the anomalycoordinate system and other data required for use by the program modules44, 46, and 48. These program modules 44, 46, and 48, as well as thestored data 42, will be further described below in conjunction withembodiments of the method of the invention.

Generally, program modules include routines, programs, objects,components, data structures, etc. that perform particular tasks orimplement particular abstract data types. Moreover, those skilled in theart will appreciate that the invention may be practiced with othercomputer system configurations, including hand-held devices,multiprocessor systems, microprocessor-based or programmable consumerelectronics, minicomputers, mainframe computers, and the like. Theinvention may also be practiced in distributed computing environmentswhere tasks are performed by remote processing devices that are linkedthrough a communications network. In a distributed computingenvironment, program modules may be located in both local and remotecomputer storage media including memory storage devices.

Although the computing system 10 is shown as having a generalized memory40, the computing system 10 would typically includes a variety ofcomputer readable media. By way of example, and not limitation, computerreadable media may comprise computer storage media and communicationmedia. The computing system memory 40 may include computer storage mediain the form of volatile and/or nonvolatile memory such as a read onlymemory (ROM) and random access memory (RAM). A basic input/output system(BIOS), containing the basic routines that help to transfer informationbetween elements within computer 10, such as during start-up, istypically stored in ROM. The RAM typically contains data and/or programmodules that are immediately accessible to and/or presently beingoperated on by processing unit 30. By way of example, and notlimitation, the computing system 10 includes an operating system,application programs, other program modules, and program data.

The components shown in the memory 40 may also be included in otherremovable/nonremovable, volatile/nonvolatile computer storage media. Forexample only, a hard disk drive may read from or write to nonremovable,nonvolatile magnetic media, a magnetic disk drive may read from or writeto a removable, non-volatile magnetic disk, and an optical disk drivemay read from or write to a removable, nonvolatile optical disk such asa CD ROM or other optical media. Other removable/non-removable,volatile/non-volatile computer storage media that can be used in theexemplary operating environment include, but are not limited to,magnetic tape cassettes, flash memory cards, digital versatile disks,digital video tape, solid state RAM, solid state ROM, and the like. Thedrives and their associated computer storage media discussed above andillustrated in FIG. 1, provide storage of computer readableinstructions, data structures, program modules and other data for thecomputing system 10.

A user may enter commands and information into the computing system 10through input devices such as a keyboard and pointing device, commonlyreferred to as a mouse, trackball or touch pad. Input devices mayinclude a microphone, joystick, satellite dish, scanner, or the like.These and other input devices are often connected to the processing unit30 through the operator interface 32 that is coupled to the system bus50, but may be connected by other interface and bus structures, such asa parallel port or a universal serial bus (USB). A monitor or other typeof display device may be connected to the system bus 50 via aninterface, such as a video interface. In addition to the monitor,computers may also include other peripheral output devices such asspeakers and printer, which may be connected through an outputperipheral interface.

Although many other internal components of the computing system 10 arenot shown, those of ordinary skill in the art will appreciate that suchcomponents and the interconnection are well known. Accordingly,additional details concerning the internal construction of the computer10 need not be disclosed in connection with the present invention.

FIG. 2 is a flow chart illustrating the procedures involved in a methodof the invention. Generally, in procedure A, the transmitters 16transmit electromagnetic signals. In procedure B, the receivers 18receive transient responses. In procedure C, the system processes thetransient responses to determine a distance and direction to theanomaly.

FIGS. 3-6 illustrate the technique for implementing procedure C fordetermining distance and direction to the anomaly.

Tri-axial Transient EM Responses

FIG. 3 illustrates directional angles between tool coordinates andanomaly coordinates. A transmitter coil T is located at an origin thatserves as the origin for each coordinate system. A receiver R is placedat a distance L from the transmitter. An earth coordinate system,includes a Z-axis in a vertical direction and an X-axis and a Y-axis inthe East and the North directions, respectively. The deviated boreholeis specified in the earth coordinates by a deviation angle θ_(b) and itsazimuth angle φ_(b). A resistivity anomaly A is located at a distance Dfrom the transmitter in the direction specified by a dip angle (θ_(a))and its azimuth (φ_(a)).

In order to practice embodiments of the method, FIG. 4A shows thedefinition of a tool/borehole coordinate system having x, y, and z axes.The z-axis defines the direction from the transmitter T to the receiverR. The tool coordinates in FIG. 4A are specified by rotating the earthcoordinates (X, Y, Z) in FIG. 3 by the azimuth angle (φ_(b)) around theZ-axis and then rotating by θ_(b) around the y-axis to arrive at thetool coordinates (x, y, z). The direction of the anomaly is specified bythe dip angle (θ) and the azimuth angle (φ) where:cos θ=({circumflex over (b)} _(z) ·â)=cos θ_(a)cos θ_(b)+sin θ_(a) sinθ_(b) cos (φ_(a)−φ_(b))

$\begin{matrix}{{\tan\;\phi} = \frac{\sin\;\theta_{b}{\sin\left( {\varphi_{a} - \varphi_{b}} \right)}}{{\cos\;\theta_{a}\sin\;\theta_{b}{\cos\left( {\varphi_{a\;} - \varphi_{b\;}} \right)}} - {\sin\;\theta_{a}\cos\;\theta_{b}}}} & (2)\end{matrix}$

Similarly, FIG. 4B shows the definition of an anomaly coordinate systemhaving a, b, and c axes. The c-axis defines the direction from thetransmitter T to the center of the anomaly A. The anomaly coordinates inFIG. 4B are specified by rotating the earth coordinates (X, Y, Z) inFIG. 3 by the azimuth angle (φ_(a)) around the Z-axis and subsequentlyrotating by θ_(a) around the b-axis to arrive at the anomaly coordinates(a, b, c). In this coordinate system, the direction of the borehole isspecified in a reverse order by the azimuth angle (φ) and the dip angle(θ).

Transient Responses in Two Coordinate Systems

The method is additionally based on the relationship between thetransient responses in two coordinate systems. The magnetic fieldtransient responses at the receivers [R_(x), R_(y), R_(z)] which areoriented in the [x, y, z] axis direction of the tool coordinates,respectively, are noted as

$\begin{matrix}{\begin{bmatrix}V_{xx} & V_{xy} & V_{xz} \\V_{yx} & V_{yy} & V_{yz} \\V_{zx} & V_{zy} & V_{zz}\end{bmatrix} = {\begin{bmatrix}R_{x} \\R_{y} \\R_{z}\end{bmatrix}\begin{bmatrix}M_{x} & M_{y} & M_{z}\end{bmatrix}}} & (3)\end{matrix}$

-   -   from a magnetic dipole source in each axis direction, [M_(x),        M_(y), M_(z)].

When the resistivity anomaly is distant from the tool, the formationnear the tool is seen as a homogeneous formation. For simplicity, themethod may assume that the formation is isotropic. Only three non-zerotransient responses exist in a homogeneous isotropic formation. Theseinclude the coaxial response and two coplanar responses. Coaxialresponse V_(zz)(t) is the response when both the transmitter and thereceiver are oriented in the common tool axis direction. Coplanarresponses, V_(xx)(t) and V_(yy)(t), are the responses when both thetransmitter T and the receiver R are aligned parallel to each other buttheir orientation is perpendicular to the tool axis. All of thecross-component responses are identically zero in a homogeneousisotropic formation. Cross-component responses are either from alongitudinally oriented receiver with a transverse transmitter, or viseversa. Another cross-component response is also zero between a mutuallyorthogonal transverse receiver and transverse transmitter.

The effect of the resistivity anomaly is seen in the transient responsesas time increases. In addition to the coaxial and the coplanarresponses, the cross-component responses V_(ij)(t) (i≠j; i, j=x, y, z)become non-zero.

The magnetic field transient responses may also be examined in theanomaly coordinate system. The magnetic field transient responses at thereceivers [R_(a), R_(b), R_(c)] that are oriented in the [a, b, c] axisdirection of the anomaly coordinates, respectively, may be noted as

$\begin{matrix}{\begin{bmatrix}V_{aa} & V_{ab} & V_{ac} \\V_{ba} & V_{bb} & V_{bc} \\V_{ca} & V_{cb} & V_{cc}\end{bmatrix} = {\begin{bmatrix}R_{a} \\R_{b} \\R_{c}\end{bmatrix}\begin{bmatrix}M_{a} & M_{b} & M_{c}\end{bmatrix}}} & (4)\end{matrix}$from a magnetic dipole source in each axis direction, [M_(a), M_(b),M_(c)].

When the anomaly is large and distant compared to thetransmitter-receiver spacing, the effect of spacing can be ignored andthe transient responses can be approximated with those of the receiversnear the transmitter. Then, the method assumes that axial symmetryexists with respect to the c-axis that is the direction from thetransmitter to the center of the anomaly. In such an axially symmetricconfiguration, the cross-component responses in the anomaly coordinatesare identically zero in time-domain measurements.

$\begin{matrix}{\begin{bmatrix}V_{aa} & V_{ab} & V_{ac} \\V_{ba} & V_{bb} & V_{bc} \\V_{ca} & V_{cb} & V_{cc}\end{bmatrix} = \begin{bmatrix}V_{aa} & 0 & 0 \\0 & V_{aa} & 0 \\0 & 0 & V_{cc}\end{bmatrix}} & (5)\end{matrix}$

The magnetic field transient responses in the tool coordinates arerelated to those in the anomaly coordinates by a simple coordinatetransformation P(θ, φ) specified by the dip angle (θ) and azimuth angle(φ).

$\begin{matrix}{\begin{bmatrix}V_{xx} & V_{xy} & V_{xz} \\V_{yx} & V_{yy} & V_{yz} \\V_{zx} & V_{zy} & V_{zz}\end{bmatrix} = {{{P\left( {\vartheta,\phi} \right)}^{tr}\begin{bmatrix}V_{aa} & V_{ab} & V_{ac} \\V_{ba} & V_{bb} & V_{bc} \\V_{ca} & V_{cb} & V_{cc}\end{bmatrix}}{P\left( {\vartheta,\phi} \right)}}} & (6) \\{{P\left( {\vartheta,\phi} \right)}\begin{bmatrix}{\cos\;\vartheta\;\cos\;\phi} & {\cos\;\vartheta\;\sin\;\phi} & {{- \sin}\;\vartheta} \\{{- \sin}\;\phi} & {\cos\;\phi} & 0 \\{\sin\;\vartheta\;\cos\;\phi} & {\sin\;\vartheta\;\sin\;\phi} & {\cos\;\vartheta}\end{bmatrix}} & (7)\end{matrix}$Determination of Target Direction

The assumptions set forth above contribute to determination of targetdirection, which is defined as the direction of the anomaly from theorigin. When axial symmetry in the anomaly coordinates is assumed, thetransient response measurements in the tool coordinates are constrainedand the two directional angles may be determined by combinations oftri-axial responses.

$\begin{matrix}{\begin{bmatrix}V_{xx} & V_{xy} & V_{xz} \\V_{yx} & V_{yy} & V_{yz} \\V_{zx} & V_{zy} & V_{zz}\end{bmatrix} = {{{P\left( {\vartheta,\phi} \right)}^{tr}\begin{bmatrix}V_{aa} & 0 & 0 \\0 & V_{aa} & 0 \\0 & 0 & V_{cc}\end{bmatrix}}{P\left( {\vartheta,\phi} \right)}}} & (8)\end{matrix}$

In terms of each tri-axial responseV _(xx)=(V _(aa) cos² θ+V _(cc) sin²θ)cos² φ+V _(aa) sin² φV _(yy)=(V _(aa) cos² θ+V _(cc)sin²θ)sin² φ+V _(aa) cos² φV _(zz) =V _(aa)sin² θ+V _(cc)cos²θ  (9)V _(xy) =V _(yx)=−(V _(aa) −V _(cc))sin² θ cos φ sin φV _(zx) =V _(xz)=−(V _(aa) −V _(cc))cos θ sin φ cos φV _(yz) =V _(zy)=−(V _(aa) −V _(cc))cos θ sin φ sin φ  (10)

The following relations can be noted:V _(xx) +V _(yy) +V _(zz)=2V _(aa) +V _(cc)V _(xx) −V _(yy)=(V _(cc) −V _(aa))sin² θ(cos² φ−sin² φ)V _(yy) −V _(zz)=−(V _(cc) −V _(aa))(cos² θ−sin² θ sin² φ)V _(zz) −V _(xx)=(V _(cc) −V _(aa))(cos² θ−sin² θ cos² φ)

Several distinct cases can be noted. In the first of these cases, whennone of the cross-components is zero, V_(xy)≠0 nor V_(yz)≠0 norV_(zx)≠0, then the azimuth angle φ is not zero nor π/2 (90°), and can bedetermined by,

$\begin{matrix}\begin{matrix}{\phi = {\frac{1}{2}\tan^{- 1}\frac{V_{xy} + V_{yx}}{V_{xx} - V_{yy}}}} \\{\phi = {{\tan^{- 1}\frac{V_{yz}}{V_{xz}}} = {\tan^{- 1}\frac{V_{zy}}{V_{zx}}}}}\end{matrix} & (12)\end{matrix}$

By noting the relation,

$\begin{matrix}{\frac{V_{xy}}{V_{xz}} = {{\tan\;\vartheta\;\sin\;\phi\mspace{14mu}{and}\mspace{14mu}\frac{V_{xy}}{V_{yz}}} = {\tan\;\vartheta\;\cos\;\phi}}} & (13)\end{matrix}$

-   -   the dip (deviation) angle θ is determined by,

$\begin{matrix}{{\tan\;\vartheta} = \sqrt{\left( \frac{V_{xy}}{V_{xz}} \right)^{2} + \left( \frac{V_{xy}}{V_{yz}} \right)^{2}}} & (14)\end{matrix}$

In the second case, when V_(xy)=0 and V_(yz)=0, then θ=0 or φ=0 or π(180°) and φ=±π/2 (90°), as the coaxial and the coplanar responsesshould differ from each other (V_(aa)≠V_(cc)).

If φ=0, then the dip angle θ is determined by,

$\begin{matrix}{\vartheta = {{- \frac{1}{2}}\tan^{- 1}\frac{V_{xz} + V_{zx}}{V_{xx} - V_{zz}}}} & (15)\end{matrix}$

If φ=π(180°), then the dip angle θ is determined by,

$\begin{matrix}{\vartheta = {{+ \frac{1}{2}}\tan^{- 1}\frac{V_{xz} + V_{zx}}{V_{xx} - V_{zz}}}} & (16)\end{matrix}$

Also, with regard to the second case, If θ=0, then V_(xx)=V_(yy) andV_(zx)=0. If φ=±π/2 (90°) and θ=±π/2 (90°), then V_(zz)=V_(xx) andV_(zx)=0. These instances are further discussed below with relation tothe fifth case.

In the third case, when V_(xy)=0 and V_(xz)=0, then φ=±π/2 (90°) or θ=0or φ=0 and θ=±π/2 (90°).

If φ=π/2, then the dip angle θ is determined by,

$\begin{matrix}{\vartheta = {{- \frac{1}{2}}\tan^{- 1}\frac{V_{yz} + V_{zy}}{V_{yy} - V_{zz}}}} & (17)\end{matrix}$

If φ=−π/2, the dip angle θ is determined by,

$\begin{matrix}{\vartheta = {{+ \frac{1}{2}}\tan^{- 1}\frac{V_{yz} + V_{zy}}{V_{yy} - V_{zz}}}} & (18)\end{matrix}$

Also with regard to the third case, If θ=0, then V_(xx)=V_(yy) andV_(yz)=0. If φ=0 and θ=±π/2 (90°), V_(yy)=V_(zz) and V_(yz)=0. Thesesituations are further discussed below with relation to the fifth case.

In the fourth case, V_(xz)=0 and V_(yz)=0, then θ=0 or π(180°) or ±π/2(90°

If θ=±π/2, then the azimuth angle φ is determined by,

$\begin{matrix}{\phi = {{- \frac{1}{2}}\tan^{- 1}\frac{V_{xy} + V_{yx}}{V_{xx} - V_{yy}}}} & (19)\end{matrix}$

Also with regard to the fourth case, if θ=0 or π(180°), thenV_(xx)=V_(yy) and V_(yz)=0. This situation is also shown below withrelation to the fifth case.

In the fifth case, all cross components vanish, V_(xz)=V_(yz)=V_(xy)=0,then θ=0, or θ=±π/2 (90°) and φ=0 or ±π/2 (90°).

If V_(xx)=V_(yy) then θ=0 or π(180°).

If V_(yy)=V_(zz) then θ=±π/2 (90°) and φ=0.

If V_(zz)=V_(xx) then θ=±π/2 (90°) and φ=±π/2 (90°).

Tool Rotation Around the Tool/Borehole Axis

In the above analysis, all the transient responses V_(ij)(t) (i, j=x, y,z) are specified by the x-, y-, and z-axis directions of the toolcoordinates. However, the tool rotates inside the borehole and theazimuth orientation of the transmitter and the receiver no longercoincides with the x- or y-axis direction as shown in FIG. 5. If themeasured responses are {tilde over (V)}_(ĩ{tilde over (j)})(ĩ, {tildeover (j)}={tilde over (x)}, {tilde over (y)}, z) where {tilde over (x)}and {tilde over (y)} axis are the direction of antennas fixed to therotating tool, and ψ is the tool's rotation angle, then

$\begin{matrix}{\begin{bmatrix}V_{\overset{\sim}{x}\overset{\sim}{x}} & V_{\overset{\sim}{x}\overset{\sim}{y}} & V_{\overset{\sim}{x}z} \\V_{\overset{\sim}{y}\overset{\sim}{x}} & V_{\overset{\sim}{y}\overset{\sim}{y}} & V_{\overset{\sim}{y}z} \\V_{z\overset{\sim}{x}} & V_{z\overset{\sim}{y}} & V_{zz}\end{bmatrix} = {{{R(\psi)}^{tr}\begin{bmatrix}V_{xx} & V_{xy} & V_{xz} \\V_{yx} & V_{yy} & V_{yz} \\V_{zx} & V_{zy} & V_{zz}\end{bmatrix}}{R(\psi)}}} & (20) \\{{R(\psi)} = \begin{bmatrix}{\cos\;\psi} & {{- \sin}\;\psi} & 0 \\{\sin\;\psi} & {\cos\;\psi} & 0 \\0 & 0 & 1\end{bmatrix}} & (21)\end{matrix}$

Then,V _({tilde over (x)}{tilde over (x)})=(V _(aa) cos² θ+V _(cc) sin²θ)cos²(φ−ψ) +V _(aa) sin² (φ−ψ)V _({tilde over (y)}{tilde over (y)})=(V _(aa) cos² θ+V _(cc) sin²θ)cos²(φ−ψ) +V _(aa) cos²(φ−ψ)V _(zz) =V _(aa) sin² θ+V _(cc) cos² θ  (22)V _({tilde over (x)}{tilde over (y)}) =V_({tilde over (y)}{tilde over (x)})=−(V _(aa) −V _(cc))sin² θ cos(φ−ψ)sin(φ−ψ)V _(z{tilde over (x)}) =V _({tilde over (x)}z)=−(V _(aa) −V _(cc))cos θsin θ cos(φ−ψ)V _({tilde over (y)}z) =V _(z{tilde over (y)})=−(V _(aa) −V _(cc))cos θsin θ sin(φ−ψ)

The following relations apply:V _({tilde over (x)}{tilde over (x)}) +V_({tilde over (y)}{tilde over (y)}) +V _(zz)=2V _(aa) +V _(cc)V _({tilde over (x)}{tilde over (x)}) −V_({tilde over (y)}{tilde over (y)})=(V _(cc) −V _(aa))sin²θ{cos²(φ−ψ)−sin²(φ−ψ)}V _({tilde over (y)}{tilde over (y)}) −V _(zz)=−(V _(cc) −V _(aa)){cos²θ−sin² θ sin ²(φ−ψ)}V _(zz) −V _({tilde over (x)}{tilde over (x)})=(V _(cc) −V _(aa)){cos²θ−sin² θ sin ²(φ−ψ)}

Consequently,

$\begin{matrix}\begin{matrix}{{\phi - \psi} = {\frac{1}{2}\tan^{- 1}\frac{V_{\overset{\sim}{x}\overset{\sim}{y}} + V_{\overset{\sim}{y}\overset{\sim}{x}}}{V_{\overset{\sim}{x}\overset{\sim}{x}} - V_{\overset{\sim}{y}\overset{\sim}{y}}}}} \\{{\phi - \psi} = {{\tan^{- 1}\frac{V_{\overset{\sim}{y}z}}{V_{\overset{\sim}{x}z}}} = {\tan^{- 1}\frac{V_{z\overset{\sim}{y}}}{V_{z\overset{\sim}{x}}}}}}\end{matrix} & (25)\end{matrix}$

The azimuth angle φ is measured from the tri-axial responses if the toolrotation angle ψ is known. To the contrary, the dip (deviation) angle θis determined by

$\begin{matrix}{{\tan\;\vartheta} = \sqrt{\left( \frac{V_{\overset{\sim}{x}\overset{\sim}{y}}}{V_{\overset{\sim}{x}z}} \right)^{2} + \left( \frac{V_{\overset{\sim}{x}\overset{\sim}{y}}}{V_{\overset{\sim}{y}z}} \right)^{2}}} & (26)\end{matrix}$without knowing the tool orientation ψ.Apparent Dip Angle and Azimuth Angle and the Distance to the Anomaly

The dip and the azimuth angle described above indicate the direction ofa resistivity anomaly determined by a combination of tri-axial transientresponses at a time (t) when the angles have deviated from a zero value.When t is small or close to zero, the effect of such anomaly is notapparent in the transient responses as all the cross-component responsesare vanishing. To identify the anomaly and estimate not only itsdirection but also the distance, it is useful to define the apparentazimuth angle φ_(app)(t) by,

$\begin{matrix}\begin{matrix}{{\phi_{app}(t)} = {\frac{1}{2}\tan^{- 1}\frac{{V_{xy}(t)} + {V_{yx}(t)}}{{V_{xx}(t)} - {V_{yy}(t)}}}} \\{{\phi_{app}(t)} = {{\tan^{- 1}\frac{V_{yz}(t)}{V_{xz}(t)}} = {\tan^{- 1}\frac{V_{zy}(t)}{V_{zx}(t)}}}}\end{matrix} & (27)\end{matrix}$and the effective dip angle θ_(app)(t) by

$\begin{matrix}{{\tan\;{\vartheta_{app}(t)}} = \sqrt{\left( \frac{V_{xy}(t)}{V_{xz}(t)} \right)^{2} + \left( \frac{V_{xy}(t)}{V_{yz}(t)} \right)^{2}}} & (28)\end{matrix}$

-   -   for the time interval when φ_(app)(t)≠0 nor π/2 (90°). For        simplicity, the case examined below is one in which none of the        cross-component measurements is identically zero: V_(xy)(t)≠0,        V_(yz)(t)≠0, and V_(zx)(t)≠0.

For the time interval when φ_(app)(t)=0, θ_(app)(t) is defined by,

$\begin{matrix}{{\vartheta_{app}(t)} = {{- \frac{1}{2}}\tan^{- 1}\frac{{V_{xz}(t)} + {V_{zx}(t)}}{{V_{xx}(t)} - {V_{zz}(t)}}}} & (29)\end{matrix}$

For the time interval when φ_(app)(t)=π/2 (90°), θ_(app)(t) is definedby,

$\begin{matrix}{{{\vartheta\;}_{app}(t)} = {{- \frac{1}{2}}\tan^{- 1}\frac{{V_{yz}(t)} + {V_{zy}(t)}}{{V_{yy}(t)} - {V_{zz}(t)}}}} & (30)\end{matrix}$

When t is small and the transient responses do not see the effect of aresistivity anomaly at distance, the effective angles are identicallyzero, φ_(app)(t)=θ_(app)(t)=0. As t increases, when the transientresponses see the effect of the anomaly, φ_(app)(t) and θ_(app)(t) beginto show the true azimuth and the true dip angles. The distance to theanomaly may be indicated at the time when φ_(app)(t) and θ_(app)(t)start deviating from the initial zero values. As shown below in amodeling example, the presence of an anomaly is detected much earlier intime in the effective angles than in the apparent conductivity(σ_(app)(t)). Even if the resistivity of the anomaly may not be knownuntil σ_(app)(t) is affected by the anomaly, its presence and thedirection can be measured by the apparent angles. With limitation intime measurement, the distant anomaly may not be seen in the change ofσ_(app)(t) but is visible in φ_(app)(t) and θ_(app)(t).

Modeling Example

A simplified modeling example exists when a resistivity anomaly is amassive salt dome, and the salt interface may be regarded as a planeinterface. For further simplification, it can be assumed that theazimuth of the salt face is known. Accordingly, the remaining unknownsare the distance D to the salt face from the tool, the isotropic oranisotropic formation resistivity, and the approach angle (or dip angle)θ as shown in FIG. 6.

Table 1 and Table 2 below show the voltage from the coaxial V_(zz)(t),coplanar V_(xx)(t), and the cross-component V_(zx)(t) measurements forL=1 m, for θ=30° and at salt distance D=10 m and D=100 m respectively.The apparent dip θ_(app)(t) is defined by,

$\begin{matrix}{{\theta_{app}(t)} = {{- \frac{1}{2}}\tan^{- 1}{\frac{{V_{zx}(t)} + {V_{xz}(t)}}{{V_{zz}(t)} - {V_{xx}(t)}}.}}} & (31)\end{matrix}$

Table 3 below shows the apparent dip (θ_(app)(t)) for the L=1 m toolassembly when the salt face is D=10 m away and at the approach angle ofθ=30°.

In addition, the apparent conductivity (σ_(app)(t)) from both thecoaxial (V_(zz)(t)) and the coplanar (V_(xx)(t)) responses is shown inTable 4, wherein the approach angle (θ) and salt face distance (D) arethe same as in Table 3.

Also plotted is the ratio, σ_(app-coplanar)(t)/σ_(app-coaxial)(t), thatis available without cross-component V_(zx)(t) measurements as shown inTable 5, wherein the approach angle (θ) and salt face distance (D) arethe same as in FIG. 3.

Note that the direction to the salt face is immediately identified inthe apparent dip θ_(app)(t) plot of Table 3 as early as 10⁻⁴ second whenthe presence of the resistivity anomaly is barely detected in theapparent conductivity (σ_(app)(t)) plot of Table 4. It takes almost 10⁻³second for the apparent conductivity to approach an asymptoticσ_(app)(later t) value and for the apparent conductivity ratio to readθ=30°.

Table 6 below shows the apparent dip θ_(app)(t) for the L=1 m toolassembly when the salt face is D=10 m away, but at different anglesbetween the tool axis and the target. The approach angle (θ) may beidentified at any angle.

Tables 7, 8, and 9 below compare the apparent dip θ_(app)(t) fordifferent salt face distances (D) and different angles between the toolaxis and the target.

The distance to the salt face can be also determined by the transitiontime at which θ_(app)(t) takes an asymptotic value. Even if the saltface distance (D) is 100 m, it can be identified and its direction canbe measured by the apparent dip θ_(app)(t).

In summary, the method considers the coordinate transformation oftransient EM responses between tool-fixed coordinates and anomaly-fixedcoordinates. When the anomaly is large and far away compared to thetransmitter-receiver spacing, one may ignore the effect of spacing andapproximate the transient EM responses with those of the receivers nearthe transmitter. Then, one may assume axial symmetry exists with respectto the c-axis that defines the direction from the transmitter to theanomaly. In such an axially symmetric configuration, the cross-componentresponses in the anomaly-fixed coordinates are identically zero. Withthis assumption, a general method is provided for determining thedirection to the resistivity anomaly using tri-axial transient EMresponses.

The method defines the apparent dip θ_(app)(t) and the apparent azimuthφ_(app)(t) by combinations of tri-axial transient measurements. Theapparent direction {θ_(app)(t), φ_(app)(t)} reads the true direction {θ,φ} at later time. The θ_(app)(t) and φ_(app)(t) both read zero when t issmall and the effect of the anomaly is not sensed in the transientresponses or the apparent conductivity. The conductivities(σ_(coaxial)(t) and σ_(coplanar)(t)) from the coaxial and coplanarmeasurements both indicate the conductivity of the near formation aroundthe tool.

Deviation of the apparent direction ({θ_(app)(t), φ_(app)(t)}) from zeroidentifies the anomaly. The distance to the anomaly is measured by thetime when the apparent direction ({θ_(app)(t), φ_(app)(t)}) approachesthe true direction ({θ, φ}). The distance can be also measured from thechange in the apparent conductivity. However, the anomaly is identifiedand measured much earlier in time in the apparent direction than in theapparent conductivity.

Apparent Conductivity

As set forth above, apparent conductivity can be used as an alternativetechnique to apparent angles in order to determine the location of ananomaly in a wellbore. The time-dependent apparent conductivity can bedefined at each point of a time series at each logging depth. Theapparent conductivity at a logging depth z is defined as theconductivity of a homogeneous formation that would generate the sametool response measured at the selected position.

In transient EM logging, transient data are collected at a logging depthor tool location z as a time series of induced voltages in a receiverloop. Accordingly, time dependent apparent conductivity (σ(z; t)) may bedefined at each point of the time series at each logging depth, for aproper range of time intervals depending on the formation conductivityand the tool specifications.

Apparent Conductivity for a Coaxial Tool

The induced voltage of a coaxial tool with transmitter-receiver spacingL in the homogeneous formation of conductivity (σ) is given by,

$\begin{matrix}{{V_{zZ}(t)} = {{C\frac{\left( {\mu_{0}\sigma} \right)^{\frac{3}{2}}}{{8_{t}}^{\frac{5}{2}}}{\mathbb{e}}^{- u^{2}}\mspace{14mu}{where}\mspace{14mu} u^{2}} = {\frac{{\mu_{0}\sigma}\;}{4}\frac{L^{2}}{t}\mspace{14mu}{and}\mspace{14mu} C\mspace{14mu}{is}\mspace{14mu} a\mspace{14mu}{{constant}.}}}} & (32)\end{matrix}$

FIG. 7 illustrates a coaxial tool in which both a transmitter coil (T)and a receiver coil (R) are wound around the common tool axis. Thesymbols σ₁ and σ₂ may represent the conductivities of two formationlayers. This tool is used to illustrate the voltage response fordifferent values of t and L in Tables 10-12 below, where σ₁=σ₂.

Table 10 shows the voltage response of the coaxial tool with L=01 m in ahomogeneous formation for various formation resistivities (R) from 1000ohm-m to 0.1 ohm-m. The voltage is positive at all times t for t>0. Theslope of the voltage is nearly constant

$\frac{{\partial\ln}\;{V_{zZ}(t)}}{{\partial\ln}\; t} \approx {- \frac{5}{2}}$in the time interval between 10⁻⁸ second and 1 second (and later) forany formation resistivity larger than 10 ohm-m. The slope changes signat an earlier time around 10⁻⁶ second when the resistivity is low as 0.1ohm-m.

Table 11 shows the voltage response as a function of formationresistivity at different times (t) for the same coaxial tool spacing(L=01 m). For the resistivity range from 0.1 ohm-m to 100 ohm-m, thevoltage response is single valued as a function of formation resistivityfor the measurement time (t) later than 10⁻⁶ second. At smaller times(t), for instance at 10⁻⁷ second, the voltage is no longer singlevalued. The same voltage response is realized at two different formationresistivity values.

Table 12 shows the voltage response as a function of formationresistivity for a larger transmitter-receiver spacing of L=10 m on acoaxial tool. The time interval when the voltage response is singlevalued is shifted toward larger times (t). The voltage response issingle valued for resistivity from 0.1 ohm-m to 100 ohm-m, for themeasurement time (t) later than 10⁻⁴ second. At smaller values of t, forinstance at t=10⁻⁵ second, the voltage is no longer single valued. Theapparent conductivity from a single measurement (coaxial, singlespacing) alone is not well defined.

For relatively compact transmitter-receiver spacing (L=1 m to 10 m), andfor the time measurement interval where t is greater than 10⁻⁶ second,the transient EM voltage response is mostly single valued as a functionof formation resistivity between 0.1-ohm-m and 100 ohm-m (and higher).This enables definition of the time-changing apparent conductivity fromthe voltage response (V_(zZ)(t)) at each time of measurement as:

$\begin{matrix}{{C\frac{\left( {\mu_{0}{\sigma_{app}(t)}} \right)^{\frac{3}{2}}}{8t^{\frac{5}{2}}}{\mathbb{e}}^{- {u_{app}{(t)}}^{2}}} = {V_{zZ}(t)}} & (33)\end{matrix}$where

${u_{app}(t)}^{2} = {\frac{\mu_{0}{\sigma_{app}(t)}}{4}\frac{L^{2}}{t}}$and V_(zZ)(t) on the right hand side is the measured voltage response ofthe coaxial tool. From a single type of measurement (coaxial, singlespacing), the greater the spacing L, the larger the measurement time (t)should be to apply the apparent conductivity concept. The σ_(app)(t)should be constant and equal to the formation conductivity in ahomogeneous formation: σ_(app)(t)=σ: The deviation from a constant (σ)at time (t) suggests a conductivity anomaly in the region specified bytime (t).Apparent Conductivity for a Coplanar Tool

The induced voltage of the coplanar tool with transmitter-receiverspacing L in the homogeneous formation of conductivity (σ) is given by,

$\begin{matrix}{{V_{xX}(t)} = {C\frac{\left( {\mu_{0}{\sigma_{app}(t)}} \right)^{\frac{3}{2}}}{8t^{\frac{3}{2}}}\left( {1 - u^{2}} \right)e^{- u^{2}}}} & (34)\end{matrix}$where

$u^{2} = {\frac{\mu_{0}{\sigma_{app}(t)}}{4t}L^{2}}$and C is a constant. At small values of t, the coplanar voltage changespolarity depending on the spacing L and the formation conductivity.

FIG. 8 illustrates a coplanar tool in which the transmitter (T) and thereceiver (R) are parallel to each other and oriented perpendicularly tothe tool axis. The symbols σ₁ and σ₂ may represent the conductivities oftwo formation layers. This tool is used to illustrate the voltageresponse for different values of t and L in Tables 13-14 below, whereσ₁=σ₂.

Table 13 shows the voltage response of a coplanar tool with a lengthL=01 m as a function of formation resistivity at different times (t).For the resistivity range from 0.1 ohm-m to 100 ohm-m, the voltageresponse is single valued as a function of formation resistivity forvalues of t larger than 10⁻⁶ second. At smaller values of t, forinstance at t=10⁻⁷ second, the voltage changes polarity and is no longersingle valued.

Table 14 shows the voltage response as a function of formationresistivity at different times (t) for a longer coplanar tool with alength L=05 m. The time interval when the voltage response is singlevalued is shifted towards larger values of t.

Similarly to the coaxial tool response, the time-changing apparentconductivity is defined from the coplanar tool response V_(xX)(t) ateach time of measurement as,

${C\frac{\left( {\mu_{0}{\sigma_{app}(t)}} \right)^{\frac{3}{2}}}{8t^{\frac{5}{2}}}\left( {1 - {u_{app}(t)}^{2}} \right){\mathbb{e}}^{- {u_{app}{(t)}}^{2}}} = {V_{xX}(t)}$where

${u_{app}(t)}^{2} = {\frac{\mu_{0}{\sigma_{app}(t)}}{4}\frac{L^{2}}{t}}$and V_(xX)(t) on the right hand side is the measured voltage response ofthe coplanar tool. The longer the spacing, the larger the value t shouldbe to apply the apparent conductivity concept from a single type ofmeasurement (coplanar, single spacing). The σ_(app)(t) should beconstant and equal to the formation conductivity in a homogeneousformation: σ_(app)(t)=σ.Apparent Conductivity for a Pair of Coaxial Tools

When there are two coaxial receivers, the ratio between the pair ofvoltage measurements is given by,

$\begin{matrix}{\frac{V_{zZ}\left( {L_{1};t} \right)}{V_{zZ}\left( {L_{2};t} \right)} = {\mathbb{e}}^{{- \frac{\mu_{0}\sigma}{4t}}{({L_{1}^{2} - L_{2}^{2}})}}} & (36)\end{matrix}$where L₁ and L₂ are transmitter-receiver spacing of two coaxial tools.

Conversely, the time-changing apparent conductivity is defined for apair of coaxial tools by,

$\begin{matrix}{{\sigma_{app}(t)} = {\frac{- {\ln\left( \frac{V_{zZ}\left( {L_{1};t} \right)}{V_{zZ}\left( {L_{2};t} \right)} \right)}}{\left( {L_{1}^{2} - L_{2}^{2}} \right)}\frac{4t}{\mu_{0}}}} & (37)\end{matrix}$at each time of measurement. The σ_(app)(t) should be constant and equalto the formation conductivity in a homogeneous formation: σ_(app)(t)=σ.

The apparent conductivity is similarly defined for a pair of coplanartools or for a pair of coaxial and coplanar tools. The σ_(app)(t) shouldbe constant and equal to the formation conductivity in a homogeneousformation: σ_(app)(t)=σ. The deviation from a constant (σ) at time (t)suggests a conductivity anomaly in the region specified by time (t).

Analysis of Coaxial Transient Response in Two-layer Models

To illustrate usefulness of the concept of apparent conductivity, thetransient response of a tool in a two-layer earth model, as in FIG. 7for example, can be examined. A coaxial tool with a transmitter-receiverspacing L may be placed in a horizontal well. Apparent conductivity(σ_(app)(t)) reveals three parameters including: (1) the conductivity(σ₁=0.1 S/m) of a first layer in which the tool is placed; (2) theconductivity (σ₂₌1 S/m) of an adjacent bed; and (3) the distance of thetool (horizontal borehole) to the layer boundary, d=1, 5, 10, 25, and 50m.

Under a more general circumstance, the relative direction of a boreholeand tool to the bed interface is not known. In the case of horizontalwell logging, it's trivial to infer that the tool is parallel to theinterface as the response does not change when the tool moves.

The voltage response of the L=01 m transmitter-receiver offset coaxialtool at different distances is shown in Table 15. Information can bederived from these responses using apparent conductivity as furtherexplained with regard to Table 16. Table 16 shows the voltage data ofTable 15 plotted in terms of apparent conductivity. The apparentconductivity plot shows conductivity at small t, conductivity at larget, and the transition time that moves as the distance (d) changes.

As will be further explained below, in a two-layer resistivity profile,the apparent conductivity as t approaches zero can identify the layerconductivity around the tool, while the apparent conductivity as tapproaches infinity can be used to determine the conductivity of theadjacent layer at a distance. The distance to a bed boundary from thetool can also be measured from the transition time observed in theapparent conductivity plot. The apparent conductivity plot for both timeand tool location may be used as an image presentation of the transientdata. Similarly, Table 17 illustrates the apparent conductivity in atwo-layer model where σ₁=1 S/m (R₁=1 ohm-m) and σ₂=0.1 S/m (R₂=1 ohm-m).

Conductivity at Small Values of t

At small values of t, the tool reads the apparent conductivity of thefirst layer around the tool. At large values of t, the tool reads 0.4S/m for a two-layer model where σ₁=0.1 S/m (R₁=10 ohm-m) and σ₂=1 S/m(R₂=1 ohm-m), which is an average between the conductivities of the twolayers. The change of distance (d) is reflected in the transition time.

Conductivity at small values of t is the conductivity of the local layerwhere the tool is located. At small values of t, the signal reaches thereceiver directly from the transmitter without interfering with the bedboundary. Namely, the signal is affected only by the conductivity aroundthe tool. Conversely, the layer conductivity can be measured easily byexamining the apparent conductivity at small values of t.

Conductivity at Large Values of t

Conductivity at large values of t is some average of conductivities ofboth layers. At large values of t, nearly half of the signals come fromthe formation below the tool and the remaining signals come from above,if the time for the signal to travel the distance between the tool andthe bed boundary is small.

Table 18 compares the σ_(app)(t) plot of Tables 16 and 17 for L=1 m andd=1 m where the resistivity ratio R₁/R₂ is 10:1 in Table 16 and 1:10 inTable 17. Though not shown, the conductivity at large values of t has aslight dependence on d. When the dependence is ignored, the conductivityat large values of t is determined solely by the conductivities of thetwo layers and is not affected by the location of the tool in layer 1 orlayer 2.

Table 19 compares the σ_(app)(t) plots for d=1 m but with differentspacings L. The σ_(app)(t) reaches the nearly constant conductivity atlarge values of t as L increases. However, the conductivity at largevalues of t is almost independent of the spacing L for the range of dand the conductivities considered.

Table 20 compares the σ_(app)(t) plots for d=1 m and L=1 m but fordifferent resistivity ratios. The apparent conductivity at large t isproportional to σ₁ for the same ratio (σ₁/σ₂). For instance:

Table 21 shows examples of the σ_(app)(t) plots for d=1 m and L=1 m butfor different resistivity ratios of the target layer 2 while the localconductivity (σ₁) is fixed at 1 S/m (R₁=1 ohm-m). The apparentconductivity at large values of t is determined by the target layer 2conductivity, as shown in Table 22 when σ₁ is fixed at 1 S/m.

Numerically, the late time conductivity may be approximated by thesquare root average of two-layer conductivities as:

$\begin{matrix}{\sqrt{\sigma_{app}\left( {{\left. t\rightarrow\infty \right.;\sigma_{1}},\sigma_{2}} \right)} = \frac{\sqrt{\sigma_{1}} + \sqrt{\sigma_{2}}}{2}} & (39)\end{matrix}$

To summarize, the conductivity at large values of t (as t approachesinfinity) can be used to estimate the conductivity (σ₂) of the adjacentlayer when the local conductivity (σ₁) near the tool is known, forinstance from the conductivity as t approaches 0 as illustrated in Table23.

Estimation of d, the Distance to the Adjacent Bed

The transition time at which the apparent conductivity (σ_(app)(t))starts deviating from the local conductivity (σ₁) towards theconductivity at large values of t depends on d and L, as shown in Table24.

For convenience, the transition time (t_(c)) can be defined as the timeat which the σ_(app)(t_(c)) takes the cutoff conductivity (σ_(c)). Inthis case, the cutoff conductivity is represented by the arithmeticaverage between the conductivity as t approaches zero and theconductivity as t approaches infinity. The transition time (t_(c)) isdictated by the ray path:

$\begin{matrix}\sqrt{{\left( \frac{L}{2} \right)^{2} + d^{2}},} & (40)\end{matrix}$that is the shortest distance for the EM signal traveling from thetransmitter to the bed boundary, to the receiver, independently of theresistivity of the two layers. Conversely, the distance (d) can beestimated from the transition time (t_(c)), as shown in Table 25.Other Uses of Apparent Conductivity

Similarly to conventional induction tools, the apparent conductivity(σ_(app)(z)) is useful for analysis of the error in transient signalprocessing. The effect of the noise in transient response data may beexamined as the error in the conductivity determination.

A plot of the apparent conductivity (σ_(app)(z; t)) for differentdistances (d) in both the z and t coordinates may serve as an imagepresentation of the transient data as shown in Table 26 for a L=01 mtool. The z coordinate references the tool depth along the borehole. Theσ_(app)(z; t) plot shows the approaching bed boundary as the tool movesalong the borehole.

The apparent conductivity should be constant and equal to the formationconductivity in a homogeneous formation. The deviation from a constantconductivity value at time (t) suggests the presence of a conductivityanomaly in the region specified by time (t).

Look-Ahead Capabilities of EM Transient Method

By analyzing apparent conductivity or its inherent inverse equivalent(apparent resistivity), the present invention can identify the locationof a resistivity anomaly (e.g., a conductive anomaly and a resistiveanomaly). Further, resistivity or conductivity can be determined fromthe coaxial and/or coplanar transient responses. As explained above, thedirection of the anomaly can be determined if the cross-component dataare also available. To further illustrate the usefulness of theseconcepts, the foregoing analysis may also be used to detect an anomalyat a distance ahead of the drill bit.

Analysis of Coaxial Transient Responses in Two-Layer Models

FIG. 9 shows a coaxial tool with transmitter-receiver spacing L placedin, for example, a vertical well approaching an adjacent bed that is theresistivity anomaly. The tool includes both a transmitter coil T and areceiver coil R, which are wound around a common tool axis and areoriented in the tool axis direction. The symbols σ₁ and σ₂ may representthe conductivities of two formation layers.

To show that the transient EM method can be used as a look-aheadresistivity logging method, the transient response of the tool in atwo-layer earth model may be examined. There are three parameters thatmay be determined in the two-layer model. These are: (1) theconductivity (σ₁=0.1 S/m) or resistivity (R₁=10 ohm-m) of the locallayer where the tool is placed; (2) the conductivity (σ₂=1 S/m) orresistivity (R₂=1 ohm-m) of an adjacent bed; and (3) the distance of thetool to the layer boundary, d=1, 5, 10, 25, and 50 m. Under a moregeneral circumstance, the relative direction of a borehole and tool tothe bed interface is not known.

The voltage response of the L=01 m (transmitter-receiver offset) coaxialtool at different distances (d) as a function of t is shown in Table 27.Though the difference is observed among responses at differentdistances, it is not straightforward to identify the resistivity anomalyfrom these responses.

The same voltage data of Table 27 is plotted in terms of the apparentconductivity (σ_(app)(t)) in Table 28. From this table, it is clear thatthe coaxial response can identify an adjacent bed of higher conductivityat a distance. Even a L=01 m tool can detect the bed at 10, 25-and 50-maway if low voltage response can be measured for 0.1-1 second long.

The σ_(app)(t) plot exhibits at least three parameters very distinctlyin the figure: the early time conductivity; the later time conductivity;and the transition time that moves as the distance (d) changes. In Table28, it should be noted that, at early time, the tool reads the apparentconductivity of 0.1 S/m that is of the layer just around the tool. Atlater time, the tool reads close to 0.55 S/m, an arithmetic averagebetween the conductivities of the two layers. The change of distance (d)is reflected in the transition time.

Table 29 illustrates the σ_(app)(t) plot of the coaxial transientresponse in the two-layer model of FIG. 9 for an L=01 m tool atdifferent distances (d), except that the conductivity of the local layer(σ₁) is 1 S/m (R₁=1 ohm-m) and the conductivity of the target layer (σ₂)is 0.1 S/m (R₂=10 ohm-m). Again, the tool reads at early time theapparent conductivity of 1.0 S/m that is of the layer just around thetool. At a later time, the tool reads about 0.55 S/m, the same averageconductivity value as in Table 28. The change of distance (d) isreflected in the transition time.

Early Time Conductivity (σ_(app)(t→0))

It is obvious that the early time conductivity is the conductivity ofthe local layer where the tool is located. At such an early time, thesignal reaches the receiver directly from the transmitter withoutinterfering with the bed boundary. Hence, it is affected only by theconductivity around the tool. Conversely, the layer conductivity can bemeasured easily by the apparent conductivity at an earlier time.

Late Time Conductivity (σ_(app)(t→∞))

On the other hand, the late time conductivity must be some average ofconductivities of both layers. At later time, nearly half of the signalscome from the formation below the tool and the other half from above thetool, if the time to travel the distance (d) of the tool to the bedboundary is small.

Table 30 compares the σ_(app)(t) plot of Table 28 and Table 29 for L=01m and d=01 m. The late time conductivity is determined solely by theconductivities of the two layers (σ₁ and σ₂) alone. It is not affectedby where the tool is located in the two layers. However, because of thedeep depth of investigation, the late time conductivity is not readilyreached even at t=1 second, as shown in Table 31 for the same tool. Inpractice, the late time conductivity may have to be approximated byσ_(app)(t=1 second) which slightly depends on d as illustrated in Table31.

Table 32 compares the σ_(app)(t) plots for d=1 m but with differentspacing L. The σ_(app)(t) reaches a nearly constant late timeconductivity at later times as L increases. The late time conductivity(σ_(app) (t→∞) is nearly independent of L. However, the late timeconductivity defined at t=1 second, depends on the distance (d) as shownin Table 33.

Table 34 compares the σ_(app)(t) plots for d=05 m and L=01 m but fordifferent resistivity ratios. This table shows that the late timeapparent conductivity is proportional to σ₁ for the same ratio (σ₁/σ₂).For instance:

Table 35 shows examples of the σ_(app)(t) plots for d=05 m and L=01 mbut for different resistivity ratios while the target resistivity isfixed at R₂=1 ohm-m. The late time apparent conductivity at t=1 secondis determined by the local layer conductivity as shown in Table 36.Numerically, the late time conductivity may be approximated by thearithmetic average of two-layer conductivities as:

${\sigma_{app}\left( {{\left. t\rightarrow\infty \right.;\sigma_{1}},\sigma_{2}} \right)} = {\frac{\sigma_{1} + \sigma_{2}}{2}.}$This is reasonable considering that, with the coaxial tool, the axialtransmitter induces the eddy current parallel to the bed boundary. Atlater time, the axial receiver receives horizontal current nearlyequally from both layers. As a result, the late time conductivity mustsee conductivity of both formations with nearly equal weight.

To summarize, the late time conductivity (σ_(app)(t→∞)) at t=1 secondcan be used to estimate the conductivity of the adjacent layer (σ₂) whenthe local conductivity near the tool (σ₁) is known, for instance, fromthe early time conductivity (σ_(app)(t→0)=σ₁). This is illustrated inTable 37.

Estimation of the Distance (d) to the Adjacent Bed

The transition time (t_(c)) at which the apparent conductivity startsdeviating from the local conductivity (σ₁) toward the late timeconductivity clearly depends on d, the distance of the tool to the bedboundary, as shown in Table 38 for a L=01 m tool.

For convenience, the transition time (t_(c)) is defined by the time atwhich the σ_(app)(t_(c)) takes the cutoff conductivity (σ_(c)), that is,in this example, the arithmetic average between the early time and thelate time conductivities: σ_(c)={σ_(app)(t→0)+σ_(app)(t→∞) }/2. Thetransition time (t_(c)) is dictated by the ray-path (d) minus L/2 thatis, half the distance for the EM signal to travel from the transmitterto the bed boundary to the receiver, independently on the resistivity ofthe two layers. Conversely, the distance (d) can be estimated from thetransition time (t_(c)), as shown in Table 39 when L=01 m.

Image Presentation with the Apparent Conductivity

A plot of the apparent conductivity (σ_(app)(z; t)) in both z- andt-coordinates may serve as an image presentation of the transient data,which represents apparent conductivity plots for the same tool atdifferent depths, as shown in Table 40. The z-coordinate represents thetool depth along the borehole. The σ_(app)(z; t) plot clearly helps tovisualize the approaching bed boundary as the tool moves along theborehole.

Analysis of Coplanar Transient Responses in Two-Layer Models

While the coaxial transient data were examined above, the coplanartransient data are equally useful as a look-ahead resistivity loggingmethod. FIG. 10 shows a coplanar tool with transmitter-receiver spacingL placed in a well approaching an adjacent bed that is the resistivityanomaly. On the coplanar tool, both a transmitter T (polarization) and areceiver R are oriented perpendicularly to the tool axis and parallel toeach other. The symbols σ₁ and σ₂ may represent the conductivities oftwo formation layers.

Corresponding to Table 28 for coaxial tool responses where L=01 m, theapparent conductivity (σ_(app)(t)) for the coplanar responses is plottedin Table 41 for different tool distances from the bed boundary. It isclear that the coplanar response can also identify an adjacent bed ofhigher conductivity at a distance. Even a L=01 m tool can detect the bedat 10-, 25- and 50-m away if low voltage responses can be measured for0.1-1 second long. The σ_(app)(t) plot for the coplanar responsesexhibits three parameters equally as well as for the coaxial responses.

Early Time Conductivity (σ_(app)(t→0))

It is also true for the coplanar responses that the early timeconductivity (σ_(app)(t→0)) is the conductivity of the local layer (σ₁)where the tool is located. Conversely, the layer conductivity can bemeasured easily by the apparent conductivity at earlier times.

Late Time Conductivity (σ_(app)(t→∞))

The late time conductivity (σ_(app)(t→∞)) is some average ofconductivities of both layers. The conclusions derived for the coaxialresponses apply equally well to the coplanar responses. However, thevalue of the late time conductivity for the coplanar responses is notthe same as for the coaxial responses. For coaxial responses, the latetime conductivity is close to the arithmetic average of two-layerconductivities in two-layer models. Table 42 shows the late timeconductivity (σ_(app)(t→∞)) for coplanar responses where d=05 m and L=01m but for different conductivities of the local layer while the targetconductivity is fixed at 1 S/m. Late time conductivity is determined bythe local layer conductivity, and is numerically close to the squareroot average as,

$\sqrt{\sigma_{app}\left( {{\left. t\rightarrow\infty \right.;\sigma_{1}},\sigma_{2}} \right)} = {\frac{\sqrt{\sigma_{1}} + \sqrt{\sigma_{2}}}{2}.}$

To summarize, the late time conductivity (σ_(app)(t→∞)) can be used toestimate the conductivity of the adjacent layer (σ₂) when the localconductivity near the tool (σ₁) is known, for instance, from the earlytime conductivity (σ_(app)(t→0)=σ₁). This is illustrated in Table 43.

Estimation of the Distance (d) to the Adjacent Bed

The transition time at which the apparent conductivity starts deviatingfrom the local conductivity (σ₁) toward the late time conductivityclearly depends on the distance (d) of the tool to the bed boundary, asshown in FIG. 10.

The transition time (t_(c)) may be defined by the time at which theσ_(app)(t_(c)) takes the cutoff conductivity (σ_(c)) that is, in thisexample, the arithmetic average between the early time and the late timeconductivities: σ_(c)={σ_(app)(t→0)+σ_(app)(t→∞)}/2. The transition time(t_(c)) is dictated by the ray-path (d) minus L/2 that is, half thedistance for the EM signal to travel from the transmitter to the bedboundary to the receiver, independently of the resistivity of the twolayers. Conversely, the distance (d) can be estimated from thetransition time (t_(c)), as shown in Table 44 where L=01 m.

The present invention has been described in relation to particularembodiments, which are intended in all respects to be illustrativerather than restrictive. Alternative embodiments will become apparent tothose skilled in the art to which the present invention pertains withoutdeparting from its scope.

From the foregoing, it will be seen that this invention is one welladapted to attain all the ends and objects set forth above, togetherwith other advantages, which are obvious and inherent to the system andmethod. It will be understood that certain features and sub-combinationsare of utility and may be employed without reference to other featuresand sub-combinations. This is contemplated and within the scope of theclaims.

1. A method for determining a distance to an anomaly in a formationahead of a wellbore using a device comprising a transmitter fortransmitting electromagnetic signals through the formation and areceiver for detecting responses, the method comprising: calculating oneof apparent conductivity and apparent resistivity based on a detectedresponse; monitoring the one of apparent conductivity and apparentresistivity over time; determining the distance to the anomaly when theone of apparent conductivity and apparent resistivity reaches anasymptotic value; and outputting the determined distance.
 2. The methodof claim 1, wherein calculating the one of apparent conductivity andapparent resistivity comprises evaluating one of a correspondingapparent conductivity and apparent resistivity of the formation in whichthe device is located.
 3. The method of claim 2, wherein calculating theone of apparent conductivity and apparent resistivity comprisesevaluating the corresponding one of apparent conductivity and apparentresistivity of the formation ahead of the device.
 4. The method of claim1, wherein determining the distance to the anomaly comprises determininga time in which the one of apparent conductivity and apparentresistivity begins to deviate from the corresponding one of apparentconductivity and apparent resistivity of the formation in which thedevice is located.
 5. A method for locating an anomaly in a formationahead of a logging tool using a transmitter and a receiver, the methodcomprising: transmitting an electromagnetic signal with the transmitterin the direction of the anomaly; measuring a transient voltage responseover time at the receiver; monitoring one of apparent conductivity andapparent resistivity based on the transient voltage response; locatingthe anomaly a distance ahead of the logging tool in the formation basedon a variation of the one of apparent conductivity and apparentresistivity over time; and outputting an outcome of the preceding step.6. A method for locating an anomaly in a formation ahead of a loggingtool using a transmitter and a receiver, the method comprising:transmitting an electromagnetic signal with the transmitter in thedirection of the anomaly; measuring a voltage response over time at thereceiver; calculating one of apparent conductivity and apparentresistivity over a selected time span that is less than one second;determining a time at which the one of apparent conductivity andapparent resistivity deviates from a constant value; locating theanomaly a distance ahead of the logging tool in the formation specifiedby the determined time; and outputting an outcome of the preceding step.7. The method of claim 6, wherein selecting the time span is based on adistance from the transmitter to the receiver.
 8. The method of claim 7,wherein the selected time span increases as the distance between thetransmitter and the receiver increases.
 9. The method of claim 6,wherein calculating the one of apparent conductivity and apparentresistivity is based on the measured voltage response, a transmitter toreceiver spacing, and the selected time span.
 10. The method of claim 6,wherein the anomaly comprises a conductive anomaly.
 11. The method ofclaim 6, wherein the anomaly comprises a resistive anomaly.
 12. A methodfor locating an anomaly in a formation relative to a logging tool in awellbore using a device comprising a transmitter for transmittingelectromagnetic signals through the formation and receiver for detectingresponses from the electromagnetic signals transmitted through theformation, the method comprising: calculating one of apparentconductivity and apparent resistivity based on one or more of thedetected responses; monitoring the one of apparent conductivity andapparent resistivity over time; determining the location of the anomalywhen the one of apparent conductivity and apparent resistivity reachesan asymptotic value; and outputting the location of the anomaly.
 13. Themethod of claim 12, wherein calculating the one of apparent conductivityand apparent resistivity comprises evaluating one of a correspondingconductivity and resistivity of the formation in which the device islocated.
 14. The method of claim 13, wherein calculating the one ofapparent conductivity and apparent resistivity comprises evaluating thecorresponding one of apparent conductivity and apparent resistivity ofthe formation ahead of the device.
 15. The method of claim 12, whereindetermining the location of the anomaly comprises determining a distanceto the anomaly in the formation ahead of the device.
 16. A method forlocating an anomaly relative to a logging tool in a wellbore using atransmitter and a receiver, the method comprising: transmitting anelectromagnetic signal with the transmitter in the direction of theanomaly; measuring a transient voltage response over time at thereceiver; monitoring one of apparent conductivity and apparentresistivity based on the transient voltage response; locating theanomaly based on a variation of the one of apparent conductivity andapparent resistivity over time; and outputting an outcome of thepreceding step.
 17. The method of claim 16, wherein locating the anomalycomprises determining a distance to the anomaly ahead of the wellbore.18. A method for locating an anomaly relative to a logging tool in aformation near a wellbore using a device comprising a transmitter and areceiver, the method comprising; transmitting an electromagnetic signalwith the transmitter in the direction of the anomaly; measuring avoltage response over time at the receiver; calculating one of apparentconductivity and apparent resistivity over a selected time span that isless than one second; determining a time in which the one of apparentconductivity and apparent resistivity deviates from a constant value;locating the anomaly based upon the determined time; and outputting anoutcome of the preceding step.
 19. The method of claim 18, wherein theanomaly is located a distance ahead of the wellbore.
 20. The method ofclaim 18, wherein the anomaly is located in the formation surroundingthe wellbore.
 21. The method of claim 18, wherein the anomaly comprisesa conductive anomaly.
 22. The method of claim 18, wherein the anomalycomprises a resistive anomaly.